FilenameUtils.getName function analysis

* This method will handle a file in either Unix or Windows format.
* The text after the last forward or backslash is returned.
*

      * a/b/c.txt --> c.txt
      * a.txt --> a.txt
      * a/b/c --> c
      * a/b/c/ --> ""
      * 

* The output will be the same irrespective of the machine that the code is running on.
*
* @param fileName the fileName to query, null returns null
* @return the name of the file without the path, or an empty string if none exists.
* Null bytes inside string will be removed
*/
public static String getName(final String fileName) {
// Pass in null and return null directly
if (fileName == null) {
return null;
}

// NonNul check
requireNonNullChars(fileName);

// find the last delimiter
final int index = indexOfLastSeparator(fileName);

// steal from the last delimiter to the end
return fileName.substring(index + 1);
}
2.1 Question 1: Why is NonNul check required?
2.1.1 How to check?
org.apache.commons.io.FilenameUtils#requireNonNullChars

/**
* Checks the input for null bytes, a sign of unsanitized data being passed to to file level functions.
*
* This may be used for poison byte attacks.
*
* @param path the path to check
*/
private static void requireNonNullChars(final String path) {
if (path. indexOf(0) >= 0) {
throw new IllegalArgumentException("Null byte present in file/path name. There are no "
+ "known legitimate use cases for such data, but several injection attacks may use it");
}
}
java.lang.String#indexOf(int) source code:

/**
* Returns the index within this string of the first occurrence of
* the specified character. If a character with value
* {@code ch} occurs in the character sequence represented by
* this {@code String} object, then the index (in Unicode
* code units) of the first such occurrence is returned. For
* values of {@code ch} in the range from 0 to 0xFFFF
* (inclusive), this is the smallest value k such that:
*

      * this.charAt(k) == ch
      * 

      * this.codePointAt(k) == ch
      * 

      * StringUtils. center(null, *, *) = null
      * StringUtils. center("", 4, ' ') = " "
      * StringUtils. center("ab", -1, ' ') = "ab"
      * StringUtils. center("ab", 4, ' ') = " ab "
      * StringUtils. center("abcd", 2, ' ') = "abcd"
      * StringUtils. center("a", 4, ' ') = " a "
      * StringUtils. center("a", 4, 'y') = "yayy"
      *